I have a spa with Balboa Vs501Z unit I installed about 4 years ago along with new circ pump. Power surges are common with bad winter weather, Spa started throwing GFI breaker, replaced the heater element and cured that problem. Then on start up, all goes well, priming, etc until heater switched on the spa shuts down after 5 seconds or so.

Error code I get a Dr code. Tried powering down multiple times no success. No spa techs where I live, remote location, so I have made many repairs. I am inclined to repalce the circuit board next.

Is the circulation pump coming on when you start up or is it the main pump coming on? If the circ pump is coming on and good water flow try removing the filter to see if the DR clears. If not and sensors are new and good water flow then yes may be the circuit board causing the issue.

I concur. The heater light comes solid for a short time, the rapid flickr, followed by DY code. Is there any way to check the sensors? I bought a new pair so now have two pairs. I removed the filter as well. I will check the circ pump but "see" very good flow in the spa. I have a Aqua Flow Circ master pump, looks like the impeller assembly can be diassembled for inspection. Any other ideas?